(CNN) -- U.S. troops in Afghanistan have been ordered to halt some joint operations with Afghan security forces after a spate of attacks on American and NATO troops by their local allies.
Gen. John Allen, the chief of U.S. and NATO forces there, has ordered commanders "to review force protection and tactical activities" after several instances in which uniformed Afghans have turned their guns on allied troops, Pentagon spokesman George Little said Monday.
"While some partnered operations are temporarily suspended, many continue -- and regional commanders have the authority to approve more," Little said.
More than 50 coalition troops have been killed in so-called "green-on-blue" attacks in Afghanistan in the first eight and a half months of 2012. Four Americans and two British troops were gunned down over the weekend in attacks believed to involve Afghan police.
In addition, insurgents disguised in U.S. Army uniforms carried out a coordinated assault Friday at a joint American-British base in the same region, raising concerns that the attackers had inside knowledge. That attack destroyed six AV-8B Harrier jets and left two others damaged, international forces said.
"Green-on-blue" refers to a color coding system used by the military, in which blue refers to the friendly force and green refers to allied forces. The spate of green-on-blue attacks come as American and NATO troops are training Afghan soldiers and police to maintain security within the country ahead of the planned end of allied combat operations in 2014.
In August, Allen estimated about a quarter of the attacks are being carried out by infiltrators from the Taliban, the Islamic militia that ruled most of Afghanistan before the U.S.-led invasion in 2001. An earlier Pentagon review that said about 10% were by Taliban forces that had sneaked into Afghan military and police ranks.
"It's less about the precision of 25 versus 10 than it is acknowledging that the Taliban are seeking ultimately to have some impact in the formation," Allen said.
Afghan President Hamid Karzai has blamed the attacks on foreign spy agencies hoping to undermine Afghan security institutions, but he did not specifically identify any countries.